debug/cmd/viewcore: coreview -> viewcore

I renamed the tool, but I needed to also rename what it calls itself.

Change-Id: Ie7792e919038cc0ba2e1f498a404b1a43e75c189
Reviewed-on: https://go-review.googlesource.com/100057
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>